How to Properly Design a Garden Roof

Garden Green Roof Drainage

Also called a living roof, a green roof garden partially or fully covers a building’s roof with vegetation, usually a succulent such as Sedum. Besides adding another dimension of green space, roof gardens have other benefits including extending roof life, slowing thermal transfer, providing cooling in the summer, attenuating noise level, improving surrounding air quality, water retention, reducing loads on storm sewer systems, aesthetic appeal and beneficial insect habitat and helping comply with environmental building standards.

As you plan your roof garden, it is imperative you verify that loading capacity of the roof supports the water-saturated weight of the roof garden.

Here is how to design roof garden drainage.

A properly designed roof garden begins with a clean, debris-free roof surface, waterproofed with roof membrane, subsequently overlaid with a root barrier. A root barrier is only required if waterproofing is asphalt based and therefore susceptible to root penetration. Start rolling out the barrier and begin installing at the roof drains, working outward and making sure that any overlaps shingle downward so water flows towards drains. Cut tight X- shaped openings in the barrier for the drains and protrusions.

To prevent roots from damaging rooftop waterproofing, limit the amount of overlapping but when necessary overlap at least 3ft (1m). End the barrier at the roof garden edge high enough to contain the garden growing medium.

Using a product like the puncture and oxidation-resistant DELTA®-ROOT BARRIER will protect the primary waterproofing membrane. After laying the root barrier, a water retention and drainage layer will control overflow and excessive rain fall.

John Anderson, a local Denver roofing contractor recommends using The DELTA®-FLORAXX  for superior water retention and drainage mat is specially designed for use in garden roofs.  It has octagon-shaped 20 mm dimples reinforced with ribbing that gives high compressive strength and offers a unique design that provides both water retention and drainage. The drainage layer contains  60% recycled HDPE, and is ideal for either extensive or intensive flat roof gardens covering large areas quickly.

Beginning at the garden roof edge on one side of the roof, roll out the board cup side up and cut tightly around drains and protrusions. Overlap adjoining rolls by 2 dimples.

The final layer prevents erosion of the growing medium from adversely affecting drainage capability. The DELTA®-BIOTOP is a lightweight, polypropylene geotextile created for garden roof systems to support the growing medium, enhancing water retention for the layer below. It is also root and rot resistant.

Begin at one side of the roof and lightly lay the DELTA®-BIOTOP over the drainage board terminating it at growing medium height on garden edge.  To prevent erosion of fine particles, overlap end and side flaps by 12″ (30 cm) and tightly cut drainage and protrusion openings. The geotextile fabric keeps the drainage cups free of media while allowing them to fill with water.

Before adding growing medium, add 14” wide bed of river rock at garden roof edges and around drains at a thickness minimally equal to media level.  The river rock prevents roots from growing around root barriers as well as protecting system components at perimeter from UV degradation. The river rock also acts as a natural barrier for the growing medium, which consists of highly absorptive materials like crushed brick, volcanic rock, mixed with 15% organic matter.

Add 4-5 inches of growing medium consisting of crushed bricks, volcanic rock and 15% organic matter, and then water sufficiently to fill retention cups and fully saturate media. Plant vegetation and check drains for obstructions.

After installation, maintain proper functioning with regular inspections keeping drains and river rock free of debris and overgrown vegetation.  Fertilize, weed, irrigate and replace plantings and eroded medium as necessary.

Dörken offers a complete garden roof system as well as innovative, high-performance  air and moisture barriers for commercial and residential construction sold under the DELTA® brand name. A North American manufacturer based out of Beamsville, Ontario, Dörken Products, Inc. is a subsidiary of Ewald Dörken AG, a leading European developer and manufacturer of waterproofing and drainage products sold worldwide. Dörken is known for delivering premium products while providing educational programs and full technical support. For more information, call 1-888-4DELTA4 (433-5824) or visit